1. Field of the Invention
The present invention relates to an ink, an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus
2. Description of the Related Art
An ink jet recording method is a recording method that minute droplets of an ink are applied to a recording medium such as plain paper or glossy medium, thereby forming an image, and is rapidly spread because of lowered price of an apparatus itself and improvement of recording speed. With rapid spreading of digital cameras in addition to the progress of the formation of high-quality images by the ink jet recording method, the ink jet recording method becomes general as an output method of images comparable with a silver halide photograph.
One of the problems involved in the ink jet recording method is poor image storage stability of the resulting recorded article. In general, a recorded article obtained by the ink jet recording method involves a problem that the image storage stability thereof is poor compared with a silver halide photograph. More specifically, when the recorded article is exposed to light, humidity or an environmental gas present in the air for a long period of time, there arises such a problem that a coloring material on the recorded article is deteriorated, and so change of color tone of the image or color fading is easy to occur. The light fastness of an image formed with a black ink often attends on change of color tone caused by deterioration of the coloring material, so the deterioration is particularly markedly recognized. Thus this drawback has bee recognized as a problem to be solved, and many proposals have been made therefor.
For example, proposals that additives such as an ultraviolet absorbent and an antioxidant are caused to be contained in an ink, thereby inhibiting deterioration of a coloring material have been made. However, in order to fully achieve an effect by using these additives, it is necessary to increase the content of the additives, and so such proposals have a possibility that the reliability of the resulting ink may be influenced to cause ejection failure or sticking of the ink.
In order to solve such a problem, various proposals have been made as to the idea that the light fastness of an image is improved by using an ink containing a coloring material having a structure excellent in light fastness. For example, there has been a proposal that the fastness properties of the resulting image, such as light fastness and ozone fastness, the storage stability of the resulting ink and the color tone of the image are made excellent by an ink containing a dye for which the number of aromatic conjugated π electrons not directly bonded to an azo group is regulated (see Japanese Patent Application Laid-Open No. 2005-139427). A proposal has been made as to a yellow to red dye excellent in color tone of an image, useful as a dye for toning of a black ink and excellent in light fastness and ozone fastness, and inks containing such a dye (see International Publication No. WO2006/001274 Pamphlet).
On the other hand, from the viewpoint of image quality, an image formed with a black ink is required to have a nearly neutral color tone in addition to a high optical density. Incidentally, the term “neutral color tone” as used in the present invention specifically means such a color tone that when a color tone of an image is evaluated by measuring a* and b* in the L*a*b* color space prescribed by CIE (International Commission on Illumination), these values fall within respective specific ranges. This may be described in detail in Examples, which will be described later. In order to achieve a nearly neutral color tone, such an image is required to exhibit nearly flat absorption in a visible region. On the other hand, it is difficult for an azo dye generally used as a water-soluble dye to achieve a high optical density and a nearly neutral color tone with one coloring material. Therefore, a technique of achieving the above requirement by combining a plurality of different dyes has been proposed.
For example, a proposal has been made as to the idea that an ink containing both a specific black dye with a bluish black tone, such as C.I. Food Black 1, and a magenta and/or yellow dye is used, thereby providing an image excellent in light fastness (see Japanese Patent Publication No. H08-026263). There has also been a proposal as to a black ink containing both a tetrakis-azo dye and a condensation compound of 4,4′-dinitrostilbene-2,2′-disulfonic acid with aminobenzene. It is said that an image excellent in light fastness and ozone fastness can be formed by using the black ink having such a composition (see Japanese Patent Application Laid-Open No. 2005-068416). Further, a black ink containing both a specific azo dye having a heterocyclic ring and an azo dye having a specific structure has been proposed (see Japanese Patent Application Laid-Open No. 2005-146244). Still further, a black ink containing both a dye having a specific structure, whose maximum absorption wavelength in absorption spectrum exists in a long wavelength region and a dye whose maximum absorption wavelength exists in a short wavelength region (see Japanese Patent Application Laid-Open No. 2006-282795). It is said that these inks provide images excellent in color tone, light fastness and ozone fastness.
The following proposal has been made as to the idea that a black ink is used to provide a gray image. Namely, there has been a proposal as to an ink for which the maximum absorption wavelength in an absorption spectrum of a coloring material is regulated and which has both a dye whose maximum absorption wavelength exists in a long wavelength region and a dye whose maximum absorption wavelength exists in a short wavelength region (see Japanese Patent Application Laid-Open No. 2006-028256). It is disclosed that the ink containing these dyes is used, thereby keeping a color tone of the gray image at a achromatic color, and that the color difference after a light fastness test is regulated, thereby providing a black ink excellent in light fastness.
The following proposal has been made for the purpose of inhibiting deterioration caused by humidity on a gray or black image formed by using toned inks respectively containing a black, yellow, magenta and cyan dyes. Namely, a proposal has been made as to the idea that inks respectively containing a black dye and a proper color dye are used. It is said that an image excellent in light fastness and humidity fastness is provided by using such an ink (see Japanese Unexamined Patent Publication No. 2006-526062). Specifically, an ink set of 3 inks of light gray, medium gray and dark gray inks each having properties of hue range, chroma range and lightness (L*) described below is used to form an image. The light gray ink has a hue range of from 250° to 310°, a chroma range less than 6 and a lightness (L*) from 54 to 72. The medium gray ink has a hue range of from 250° to 310°, a chroma range less than 6 and a lightness (L*) from 30 to 47. The dark gray ink has a hue range of from 150° to 270°, a chroma range less than 6 and a lightness (L*) from 10 to 18. It is described that when two or three inks of the light gray, medium gray and dark gray inks are used together to conduct recording, the gray color becomes a hue range of from 250° to 310° and a chroma range of from 2 to 6.